The present invention relates to a heating device, particularly with capacities of 0.5-15 MW, for instance for use in horticulture. Existing heating boilers in this field of application must be transportable by road on a truck, whereby the dimensions thereof are limited. Partly for this reason such heating devices are usually embodied as three-draught boilers, wherein the thermal efficiency and the pressure drop are adversely affected; the fire tube is usually narrow.
Such a multi-draught boiler is known for instance from the German Offenlegungsschrift DE-A-44 06 030.
Further known from the German Gebrauchsmuster DE-U-86 09 170 is a standing gas heating boiler, wherein pipes of a heat exchanger are partially embedded in insulation material arranged on a wall thereof, so that no condensation will form on this inner wall. This insulation material makes the construction of this known device complicated, while the upright arrangement makes it impossible to transport such a device of sufficient capacity for horticulture purposes.
It is an object of the present invention to improve existing heating devices, to obviate the above stated problems and to provide a heating boiler with a high thermal efficiency and a low pressure drop. It is a further object of the present invention to provide a heating device which can easily be scaled up, comprises a minimum number of preferably standardized components of sheet steel, which is easy to maintain and can function in disturbance-free manner. For combustion with a low NOx content and CO content the temperature of the flue gas must be optimized, particularly in the furnace. Condensation against the walls of the flue gas part is precluded as far as possible.
The present invention provides a device for heating fluid, comprising:
a first section for heating the fluid in which fuel and air are mixed and combusted;
a second section for heating the fluid which is disposed substantially in the line of the combustion section and in which a number of pipes for the fluid extend substantially transversely of the flow direction of the combustion gases; and
a third section for heating the fluid which is disposed substantially in the line of the second heating section and in which a number of pipes for the fluid extend substantially transversely of the flow direction of the combustion gases, wherein at least a number of the pipes in the third section are provided with ribs or fins enhancing the heat transfer.
The present invention therefore provides a so-called single-draught boiler which, owing to the transverse arrangement of the pipes, does not have to be much longer than the above mentioned three-draught boiler. The fire tube can take a spacious form, whereby it is suitable for burners with low NOx emission.
Although the pipes can be disposed in successive rows, which is structurally the simplest, it is also possible for the pipes to be successively disposed in offset or crosswise manner in the second and/or third heating circuit. This enhances the heat transfer and, in the case of rows of heating pipes in crosswise arrangement, a symmetrical inflow and heating of the fluid.
In a preferred embodiment baffles with flow passages are situated round the fire tube in order to strengthen the construction and enhance the flow of the fluid for heating in an annular space around the fire tube. The fire tube is hereby also cooled better.
The present invention further provides a method for operating a device for heating fluid, whereinsome of the heated fluid is pumped back to an inlet connection of the device for supplying the fluid for heating.
The method is preferably applied in a device according to the present invention. By means of pumping back some of the heated water the total heat transfer is improved and the pipes are cooled well. Pumping back hot water can ensure, also at low fire-load, that the temperature of the entering water lies above the dew point of about 60xc2x0 C.
A minimum volume flow can be ensured by switching on the pump, whereby the temperature difference over the boiler can be decreased, which causes a reduction in the thermal stresses in the construction. A maximum temperature difference of 30xc2x0 C. can be guaranteed.
In a further preferred embodiment the pipes of the second and third section are in roughly the form of a block and placed obliquely in the substantially cylindrical outer wall so that a so-called header is formed, whereby flow through the pipes takes place uniformly.
The first, second and third section are preferably fixed to each other as well as to a front and back end. By removing an annular weld on the front and back end all the internal parts can be pulled out of the substantially cylindrical outer wall.